This invention relates to a radiation dosimetry method and associated devices for carrying out the method. More particularly, this invention relates to such a method and associated apparatus which compensates for variations in temperature and amounts of a radiation sensitive material in a dosimeter.
In facilities where radioactive materials are used, for example, in hospitals where cancer patients receive radiation treatments or in blood banks where blood products are irradiated, various methods are used to quantitatively determine the radiation dose. The methods practiced include the use of thermoluminescent dosimeters (TLD's), ionization-type radiation detectors, photographic film, and radiochromic materials. TLD's are inconvenient because they require a complicated and time-consuming read-out process. Ionization-type radiation detectors are awkward and unwieldy and require a complicated setup. Photographic film requires a time-consuming chemical processing procedure before read-out. Radiochromic materials are inconvenient in current practice because the calculation of the dose requires a complex sequence of steps, subject to operator error.
U.S. Pat. No. 5,637,876 describes a radiation dosimeter, exemplarily for use in determining a level of radiation to which a patient is subjected during radiation treatment, which comprises a substrate provided with a layer of radiation sensitive material. The radiation sensitive material has an optical density which varies systematically in accordance with the degree of radiation exposure. The dosimeter may take the form of a card or a flexible substrate which is positionable on the patient or other irradiation subject and which is also positionable in, or slidable through a slot in, a dose reader which includes a reflection or transmission densitometer.
The radiation sensitive material of a radiation dosimeter may be dispersions of crystalline pentacosadyinoic acid (PCDA). Subjecting monomeric PCDA crystals to ionizing radiation results in progressive polymerization, the degree of polymerization increasing with radiation dose. The amount of polymerization (and hence, the radiation dose) can be determined by measuring either the optical density or the spectral absorption of the exposed dosimeter. However, it has been found that these parameters also vary with both the temperature of the device when measured as well as the thickness of PCDA dispersion. Maximum accuracy of dose measurement must account for the temperature and thickness effects.
Temperature corrections are commonly applied to the output of sensors of whatever type; the method is straight forward. First the response or reaction of a sensor to a specific action is calibrated at a given (reference) temperature. Second, the change in response with respect to temperature is measured over a range of temperature and input action; this step characterizes the temperature dependence of the sensor. Then, during actual use of the sensor, assuming it is not maintained at the reference temperature, both the sensor's reaction (to the action its purpose it is to sense) as well as the sensor's temperature are measured. The measured reaction is corrected for temperature effect by using the data from step two. This yields a temperature corrected reaction, i.e., a calculated value of the reaction the sensor would be expected to have for the action at hand had the sensor been at its reference temperature. Finally, the amount of action the sensor is being subjected to is calculated from the corrected reaction by using the calibration data of step one. While giving good results the method suffers from the requirement to measure the sensor's temperature.